GTK2->GTK3 was a major leap. For something like a GUI toolkit, changes and advancements are inevitable. A GTK4 port would be much less difficult, as the developer-facing changes are an order of magnitude smaller.
oh, you haven’t seen nothing yet. you know the lisp-y, hackable goodness you get in emacs? what if an init system was that hackable, and configured with a lisp? go give GNU shepherd a try.